It is well known to use momentum wheels in order to compensate for the periodic disturbing torques which act on stabilized spacecraft, particularly geostationary communication satellites. The momentum wheel is mounted on the spacecraft for rotation about an axis that is selectively oriented relative to the axes of the spacecraft, commonly orthogonal to the plane of the spacecraft's orbit.
In order to achieve active control about the axis orthogonal to the orbit plane, the speed of the momentum wheel is simply increased or decreased, the angular momentum of the wheel being sufficient to compensate for effects of disturbing torques on the spacecraft. In order to achieve active control about axes in the orbit plane or to compensate for effects of disturbing torques about such axes, the momentum wheel axis orientation must be altered.
In the past, momentum wheels of the type described above have been mounted on the spacecraft by means of a double gimbal system similar to those long used to mount compasses and the like, which employ redundancy in the drive mechanisms on each axis. This mounting arrangement is less than completely satisfactory for several reasons. The prior gimbals were relatively complex in design, were heavier than desired (an important factor in spacecraft) and were subject to reliability problems. In the event that any portion of one of the gimbals failed, the entire momentum wheel system were rendered inoperable.
Another type of tilting momentum wheel known in the art is described in a publication entitled "CMG's and Momentum Wheels," Revision 3, Publication No. 61-7223-00-03, published by Sperry Flight Systems. The momentum tilt wheel described in this publication is capable of providing three-axis attitude control and includes two linear gimbal actuators positioned around the periphery of the momentum wheel and a single center spherical ball bearing pivot which suspends the wheel. Spherical pivots are provided on each end of the actuators and thus act as hinge points at the vehicle interface and the interface with the housing which surrounds the wheel. This type of momentum wheel also suffers from a number of disadvantages, including the fact that the wheel cannot be properly positioned in the event that either of the linear actuators malfunctions.